This is a base class for implementing virtual base classes (what
some people call an abstract class). Kinda kooky. It allows you to
explicitly declare what methods are virtual and that must be
implemented by subclasses. This might seem silly, since your program
will halt and catch fire when an unimplemented virtual method is
hit anyway, but there's some benefits.

The error message is more informative. Instead of the usual "Can't
locate object method" error, you'll get one explaining that a virtual
method was left unimplemented.

Subclass authors can explicitly check to make sure they've implemented
all the necessary virtual methods. When used as part of a regression
test, it will shield against the virtual method requirements changing
out from under the subclass.

Finally, subclass authors can get an explicit list of everything
they're expected to implement.

Doesn't hurt and it doesn't slow you down.
